The ‘fracture toughness’ of a material is a property that characterizes the respective material's ability to resist fracture assuming the presence of a thin crack. It is typically determined from the ‘stress intensity factor’ K at which a thin crack in the material begins to grow. The ‘stress intensity factor’ K is generally meant to characterize the stress state near the tip of a crack (e.g. one caused by a remote load or residual stresses). Notably, the stress intensity factor can be a function of the loading that the crack is subjected to. Conventionally: ‘mode I’ loading references a normal tensile stress perpendicular to the crack; ‘mode II’ loading references an in-plane shear loading; and ‘mode III’ references out-of-plane shearing of the crack. Conventionally, as alluded to above, the fracture toughness of a material is often gleaned from the stress intensity factor at which a thin crack in the material being subjected to mode I loading begins to grow, i.e. its KIc value. KIc is typically determined experimentally and generally has the units of stress multiplied by the square root of distance.
Relatedly, the fracture toughness can also be expressed in terms of the energy per unit fracture surface area. This is typically indicated by the ‘J-integral.’ The J-integral can also be thought of as the energy required to grow a thin crack.
The fracture toughness of a material has conventionally been determined in a number of ways. For instance, ‘Charpy impact tests’ are often used to determine the amount of energy that can be absorbed during fracture, and this absorbed energy can be indicative of the overall fracture toughness of a material. Active Standard ASTM E23 discusses methods for implementing Charpy impact tests.
Similarly, ‘crack resistance curves’ or ‘R-curves’ are also conventionally derived and used to characterize the toughness of a material. R-curves illustrate a material's resistance to crack extension as a function of crack extension. Active Standard ASTM E561 discusses a standard test method for K-R curve determination.